This invention relates to a coding device for transmitting digital signals (time-sequential sampled data) obtained by sampling and quantizing analogue video signals in the form of coded signals after having reduced the number of quantized bits per group of sample data.
In the case where analogue video signals (image signals) are transmitted as digital signals which have been obtained by sampling and quantizing the analogue signals, usually seven or eight bits are believed necessary for linear quantization as the number of quantization bits per group of sample data (which may be called below also "image data"). When the image signals are digitized by this linear quantization, a transmission rate of about 100 Mbps of digital signals is necessary for signals according to the standard television system, and as to signals according to a high quality television system which has been proposed, a transmission rate which is more than twice as high as that stated above is required.
In a device for recording/reproducing magnetically the transmitted image, signals described above in the form of digital signals (hereinbelow called digital VTR), since the transmission rate is extremely high, the recording density on a tape is substantially lowered with respect to that obtained by a conventional VTR according to the analogue recording system, and therefore satisfactory recording time cannot be obtained. Further, the frequency band of the signals being dealt with is very wide, and the working speed of the digital signal processing circuit also presents a problem, giving rise to technical difficulties and a serious obstacle to wide spread acceptance of this digital VTR for home use, etc.
In order to resolve this problem, heretofore a so-called high efficiency coding method (by which image data to be transmitted are reduced by coding them so as to lower the transmission rate) has been studied. An example thereof is described in detail in an article "Processing of image digital signals" (in Japanese) by Takahiko FUKINUKE, publishdd by Nikkan Kogyo Shimbunsha (Daily Industrial Newspaper Publishing Co.).
As described in this literature (Chapter 9) a so-called differential pulse code modulation (DPCM) has been proposed and is widely known as a method for reducing the number of bits necessary per pixel data. By this method, the value of a particular pixel at any moment is predicted o the basis of values of pixels which have been already coded, and the necessary number of bits is reduced by coding the difference (error) between the predicted value and the value of the particular pixel at that moment.
According to this differential pulse code modulation, it is possible to reduce the number of bits per pixel to about four or five, which is about one half of that required according to the linear quantization method.
However the DPCM method described above has a problem which should be resolved that influences of a coding error in a transmission system propagate on other codes one after another (so-called error propagation). Further, since the feedback formality is adopted in general for the differential pulse code modulation, quantization noise is fed back and has influences on following pixels, or vibratory noise called leak contour pattern is produced, which gives rise to gradations, fluctuations, etc. of the image contour portion, deteriorates extremely the image quality and so forth. Particularly, for devices in which a high image quality is required, it was difficult to adopt the conventional DPCM method as described above and to put it to practical use.